These studies are directed at uncovering the mechanism of insulin action. Insulin rapidly alters the activity of a number of rate-limiting enzymes in metabolism. Many of these enzymes are known to be regulated by covalent phosphorylation. The present work is aimed at uncovering the pathways by which insulin alters the phosphorylation of these enzymes. Our attention has been focused primarily on a number of proteins whose phosphorylation is stimulated by insulin. We have recently succeeded in purifying one such protein subunit, Mr 123,000, from liver. Current studies involve the identification of this protein, and an examination of the kinases which mediate the insulin-stimulated phosphorylation.